predation and competition. Downstream trout populations are limited by temperature, which also mediates their predatory impacts (Shelton et al. 2018). Black bass,
on the other hand, are warm water species that invade up rivers from mainstream
source populations, and their penetration of headwaters is limited only by physical
barriers such as waterfalls or gradients (see van der Walt et al. 2016; Ellender et al.
2018). This has resulted in an increasing compression of native fish populations
between these two invaders, a situation which has resulted in the loss of more than
80% of habitat in some catchments (van der Walt et al. 2016).
17.2.1.4 Amphibians
A total of 104 species of alien amphibians globally have been assessed with EICAT
(Kumschick et al. 2017), of which 21 are alien species in South Africa (van Wilgen
and Wilson 2018). Duttaphrynus melanostictus (Asian Common Toad) is the only
one of these species with major impacts globally that also occurs as an alien in
South Africa, but there is no evidence for it having any impact in the country to date,
probably due to its lack of establishment within the country (Measey et al. 2017).
The only cases of documented impacts in the country are restricted to native species.
Xenopus laevis (African Clawed Toad), native to South Africa but traded intensively
globally, hybridises with the endemic X. gilli (Cape Platanna) (Picker 1985), but
importantly there is no evidence of introgression (Furman et al. 2017). Even though
these two species might have likely overlapped for millennia (Schreiner et al. 2013),
densities of X. leavis have probably been artificially increased in the last 400 years
(Measey et al. 2017), leading to intense competition and predation (Vogt et al. 2017;
de Villiers et al. 2016). Sclerophrys gutturalis (Guttural Toad), native to much of the
country but introduced to a peri-urban area of Cape Town (Vimercati et al. 2017),
could potentially threaten the native endangered Sclerophrys pantherina (Western
Leopard Toad), but no evidence of the extent of this threat has been reported to date
(Measey et al. 2017). Hyperolius marmoratus (Painted Reed Frog) is also native to
some parts of South Africa but has become invasive in other areas of the country that
are outside its natural range (Tolley et al. 2008; Davies et al. 2013). No studies on its
impacts on biodiversity have been conducted to date, but it is suspected that it may
impact the endemic Hyperolius horstocki (Arum Lily Frog) (Measey et al. 2017).
17.2.1.5 Birds
There are 415 alien bird species that have been assessed with EICAT at a global scale
(Evans et al. 2016), of which 37 occur as aliens in South Africa (van Wilgen and
Wilson 2018). Two species, Anas platyrhynchos (Mallard) and Pycnonotus jocosus
(Red-Whiskered Bulbul) are known to cause major impacts in their global
invasive range (Evans et al. 2016). Anas platyrhynchos threatens the genetic integrity of native congeners through hybridisation (e.g. Tracey et al. 2008). In southern
Africa A. platyrhynchos hybridises with endemic species such as Anas undulata
17 An Evaluation of the Impacts of Alien Species on Biodiversity
497
on the other hand, are warm water species that invade up rivers from mainstream
source populations, and their penetration of headwaters is limited only by physical
barriers such as waterfalls or gradients (see van der Walt et al. 2016; Ellender et al.
2018). This has resulted in an increasing compression of native fish populations
between these two invaders, a situation which has resulted in the loss of more than
80% of habitat in some catchments (van der Walt et al. 2016).
17.2.1.4 Amphibians
A total of 104 species of alien amphibians globally have been assessed with EICAT
(Kumschick et al. 2017), of which 21 are alien species in South Africa (van Wilgen
and Wilson 2018). Duttaphrynus melanostictus (Asian Common Toad) is the only
one of these species with major impacts globally that also occurs as an alien in
South Africa, but there is no evidence for it having any impact in the country to date,
probably due to its lack of establishment within the country (Measey et al. 2017).
The only cases of documented impacts in the country are restricted to native species.
Xenopus laevis (African Clawed Toad), native to South Africa but traded intensively
globally, hybridises with the endemic X. gilli (Cape Platanna) (Picker 1985), but
importantly there is no evidence of introgression (Furman et al. 2017). Even though
these two species might have likely overlapped for millennia (Schreiner et al. 2013),
densities of X. leavis have probably been artificially increased in the last 400 years
(Measey et al. 2017), leading to intense competition and predation (Vogt et al. 2017;
de Villiers et al. 2016). Sclerophrys gutturalis (Guttural Toad), native to much of the
country but introduced to a peri-urban area of Cape Town (Vimercati et al. 2017),
could potentially threaten the native endangered Sclerophrys pantherina (Western
Leopard Toad), but no evidence of the extent of this threat has been reported to date
(Measey et al. 2017). Hyperolius marmoratus (Painted Reed Frog) is also native to
some parts of South Africa but has become invasive in other areas of the country that
are outside its natural range (Tolley et al. 2008; Davies et al. 2013). No studies on its
impacts on biodiversity have been conducted to date, but it is suspected that it may
impact the endemic Hyperolius horstocki (Arum Lily Frog) (Measey et al. 2017).
17.2.1.5 Birds
There are 415 alien bird species that have been assessed with EICAT at a global scale
(Evans et al. 2016), of which 37 occur as aliens in South Africa (van Wilgen and
Wilson 2018). Two species, Anas platyrhynchos (Mallard) and Pycnonotus jocosus
(Red-Whiskered Bulbul) are known to cause major impacts in their global
invasive range (Evans et al. 2016). Anas platyrhynchos threatens the genetic integrity of native congeners through hybridisation (e.g. Tracey et al. 2008). In southern
Africa A. platyrhynchos hybridises with endemic species such as Anas undulata
17 An Evaluation of the Impacts of Alien Species on Biodiversity
497